tap the bead in question on a hard surface...like glass. Wood, stone and plastic all make different sounds. touch it on your cheek...stone is cool and plastic is warm to the touch. Plastic beads usually have a faint seam on them somewhere because they were press formed. Hope that helps!!
Thats a really good question...To me it looks like the round ones are hard plastic, The Brown ones are glass and some are a marble..I'm not for sure on them but i dont think they are all a plastic...